1. Technical Field
The invention relates to a solid-state imaging device and a method for driving the same.
2. Related Art
As solid-state imaging devices mounted in cellular phones, digital cameras, etc., there are a charge-coupled device (CCD) image sensor (hereinafter referred to as a CCD sensor) and a CMOS image sensor (hereinafter referred to as a CMOS sensor).
Further, in recent years, there has been a proposal of an MOS solid-state imaging device (hereinafter referred to as a modulation-on-substrate type sensor) that employs a threshold voltage modulation method and achieves a high image quality and a low power consumption at the same time. Such a modulation-on-substrate type sensor is disclosed as a first related art example, which will be described later.
A CCD sensor, which has a high power consumption due to a high driving voltage, achieves a correlated double sampling (CDS) function for removing noises and a so-called global electronic shutter function for taking images of rapidly moving objects with least distortion. The global electronic shutter function is a function for eliminating the distortion of images of objects by storing light-generated charge for a number of light-receiving elements that are placed two-dimensionally all at a time. Therefore, a CCD sensor generally has an advantage of excellent image quality.
On the other hand, a CMOS sensor of especially a CMOS-active pixel sensor (APS) type having a four-transistor configuration cannot achieve the global electronic shutter function but does achieve the CDS function. Further, a CMOS sensor generally has an advantage that the process cost is low with a low power consumption due to a low driving voltage. The reason why a general CMOS-APS sensor is incapable of global electronic shutter is because a CMOS-APS sensor operates in order to achieve the CDS function where noise components are read out first for each read-out line by resetting each floating diffusion, which is an electric charge-retaining region, and then signal components are read out.
Specifically, in a CMOS-APS sensor, in order to realize CDS function, each noise component is read out first by sequentially resetting transistors for transferring electric charge line by line selected, from which pixel signals are read out, and then each signal component is read out. The read-out of signal components is performed with the sequential resetting of transistors line by line selected. That is, when a rapidly moving object is imaged, the image obtained is distorted because the timing of read-out is gradually delayed from the first line to the last line.
In addition, the global electronic shutter function is technically available in a CMOS-APS sensor. In such a case, however, the transfer transistor described above is occupied for the use of the global electronic shutter function. Therefore, if the global electronic shutter function is achieved in a CMOS-APS sensor, there arises a problem that image quality is degraded due to the incapability of achieving the CDS function.
Further, in the modulation-on-substrate type sensor disclosed in the first related art example, signal components are read out first and, followed by resetting, noise components are read out. Then, the differences of the two types of signal components are outputted as pixel signals.
In the case of a modulation-on-substrate type sensor, a signal component that is read out contains a noise component that is left over after the previous reset. A noise component to be read out is the noise component that is left over after reset. However, there is no guarantee that the amount of a noise component remaining in a signal component after the previous reset is the same as the amount of a noise component remaining after the latest reset. That is, a pixel signal to be outputted only contains the previous noise component, not the latest noise component. Therefore, in the case of a modulation-on-substrate type sensor, there is a disadvantage that noises cannot be removed precisely due to the noncorrelation between the signal component and the noise component, which leads to the degradation of image quality.
Furthermore, a technique for achieving the global electronic shutter function in a modulation-on-substrate type sensor has also been proposed in a second related art example, which will be described later. In the technique according to the second related art example, all pixels are reset at a time and then pixel signals are sequentially read out line by line.
Moreover, as a method for achieving the global electronic shutter function in a CMOS-APS sensor, another solid-state imaging device that has an electric charge-retaining region under a transfer gate has been proposed in a third related art example, which will be described later.
Japanese Unexamined Patent Publication No. 2002-134729 is the first example of related art.
Japanese Unexamined Patent Publication No. 2004-87963 is the second example of related art.
Japanese Unexamined Patent Publication No. 2002-368201 is the third example of related art.
In the technique according to the second related art example, however, there still remains a problem of the incapability of precise noise removal due to the noncorrelation between the signal component and the noise component because pixel signals are read out by reading out signal components first and, followed by reset, noise components are read out second.
Further, in the proposal according to the third related art example, the transfer of electric charge between the electric charge-retaining region and the floating diffusion region via the transfer path regions between the foregoing two regions is performed by using only the potential profiles of the three regions. Therefore, a high gate voltage is required, which is not preferable in terms of power consumption. Furthermore, it is not easy to manufacture a CMOS-APS sensor having such potential profiles.